Generally, the process for manufacturing integrated circuits on a silicon wafer substrate typically involves deposition of a thin dielectric or conductive film on the wafer using oxidation or any of a variety of chemical vapor deposition processes; formation of a circuit pattern on a layer of photoresist material by photolithography; placing a photoresist mask layer corresponding to the circuit pattern on the wafer; etching of the circuit pattern in the conductive layer on the wafer; and stripping of the photoresist mask layer from the wafer. Each of these steps, particularly the photoresist stripping step, provides abundant opportunity for organic, metal and other potential circuit-contaminating particles to accumulate on the wafer surface.
In the semiconductor fabrication industry, minimization of particle contamination on semiconductor wafers increases in importance as the integrated circuit devices on the wafers decrease in size. With the reduced size of the devices, a contaminant having a particular size occupies a relatively larger percentage of the available space for circuit elements on the wafer as compared to wafers containing the larger devices of the past. Moreover, the presence of particles in the integrated circuits compromises the functional integrity of the devices in the finished electronic product. To achieve an ultraclean wafer surface, particles must be removed from the wafer, and particle-removing methods are therefore of utmost importance in the fabrication of semiconductors.
Because minimization of particles on wafers throughout the IC manufacturing process is critical, the environment within which the IC manufacturing process is carried out must be subjected to stringent controls on the presence of airborne particles which would otherwise enter the manufacturing environment from outside the facility. Currently, mini-environment based IC manufacturing facilities are equipped to control airborne particles much smaller than 1.0 μm. Accordingly, modern semiconductor manufacturing is carried out in a complex facility known as a cleanroom. The cleanroom is isolated from the outside environment and subjected to stringent controls on contaminants including airborne particles, metals, organic molecules and electrostatic discharges (ESDs), as well as on such environmental parameters as temperature, relative humidity, oxygen and vibration. Along with a sophisticated system of filters and equipment, a comprehensive and strictly-enforced set of procedures and practices are imposed on facility personnel in order to maintain a delicate balance of these clean air requirements and parameters for optimal IC fabrication.
Modern cleanrooms used in the fabrication of integrated circuits typically include one large fabrication room having a service access corridor that extends around the perimeter of the cleanroom and a main manufacturing access corridor that extends across the center of the cleanroom. Production bays, which accommodate the semiconductor fabrication tools, are located on respective sides of the main manufacturing access corridor. Outside air enters the cleanroom through an air purification system which is typically located above the ceiling of the cleanroom and includes particulate filters, typically HEPA (high-efficiency particulate air) filters. Through openings in the ceiling, the air is drawn downwardly in a continuous laminar flow path from the air purification system, through the cleanroom and into a recirculation air system through openings in the floor. The recirculation air system may turn the air over every six seconds in order to achieve ultraclean conditions during disturbances such as changes in personnel shifts. An exhaust system removes heat and chemicals generated during the fabrication processes.
FIG. 1 illustrates an example of a conventional air purification system 10 used to purify outside air 38 as the air 38 is drawn through the system 10 and into a semiconductor fabrication facility cleanroom (not shown). The air purification system 10 includes an elongated housing 12 having an intake end 14. A blower 18 draws the outside air 38 into the intake end 14 and then initially through a pre-filter 16 in the housing 12, which pre-filter 16 removes particles larger than a selected size from the air 38. The air then flows through an upstream cooling coil 20, which cools the air to a temperature at or below the dew point in such a manner that moisture in the air coalesces into water droplets 44, as shown in FIG. 2. Multiple nozzle conduits 22, each provided with multiple spray nozzles 24, are typically provided in the housing 12 and each forms a water spray 25 that generates a fine mist of additional water droplets 44. The water droplets 44 bind to the airborne particles 42 and carry the particles 42 to a high density eliminator 26, which is typically constructed of high-density paper or non-woven cloth. A downstream cooling coil 28 is provided on the opposite side of the high density eliminator 26. As shown in FIG. 2, many of the particles 42, bound to the water droplets 44, accumulate on the high density eliminator 26. Most of the droplet-bound particles 42 are eventually pulled by gravity down the high density eliminator 26 and are collected in a collection pan 46 at the bottom of the housing 12. The air, from which most of the airborne particles 42 have been removed, next flows typically through a low density eliminator 30 which is typically constructed of a low-density fabric curtain material, and then, through a heating coil 32 which heats the air typically to room temperature. Before exiting the outlet end 48 of the housing 12, the air is passed through a chemical filter 34, which removes chemical residues from the air, and finally, through a HEPA filter 36. The HEPA filter 36 is a high-efficiency filter which removes about 99.97% of the airborne particles from the air flowing therethrough. Finally, the purified air 40 emerges from the outlet end 48 of the housing 12 and enters the cleanroom (not shown) of the facility through a suitable air distribution system (not shown).
The presence of the high-density eliminator 26 inside the housing 12 induces a substantially high pressure differential in the housing 12 on respective sides of the high density eliminator 26, with the air pressure downstream of the high density eliminator 26 substantially lower than the air pressure upstream of the high density eliminator 26. This reduction in air pressure causes re-vaporization of some of the water droplets 44 at the downstream surface of the high density eliminator 26 which faces the downstream cooling coil 28. As shown in FIG. 2, re-vaporization of the water droplets 44, in turn, causes the particles 42 on the downstream surface of the high density eliminator 26 to again become airborne as the air continuously flows through the high density eliminator 26 toward the outlet end 48 of the housing 12. The smaller the quantity of water droplets 44 generated by the water sprays 25, the more readily the water droplets 44 re-vaporize at the downstream surface of the high density eliminator 26. Consequently, a large quantity of water must be used to sustain the water sprays 25 in order to form a saturation environment sufficient to prevent or substantially reduce re-vaporization of the water droplets 44 from the high density eliminator 26 and thus, reduce the quantity of particles 42 that become airborne at the downstream surface of the high density eliminator 26. Accordingly, a new and improved air purification system is needed for preventing or minimizing re-vaporization of water droplets after the water droplets capture airborne particles and adhere to a surface inside the system.
An object of the present invention is to provide a new and improved purification system for removing particles from a flowing gas.
Another object of the present invention is to provide a new and improved air purification system which is suitable for purifying outside air and introducing the air into a cleanroom for the manufacture of semiconductor integrated circuits.
Still another object of the present invention is to provide an air purification system which utilizes a water droplet trap for the adhesion of water droplets after the droplets capture airborne particles from a flowing airstream.
Yet another object of the present invention is to provide an air purification system which is efficient and reduces the quantity of water required to remove particles from a stream of flowing air.
A still further object of the present invention is to provide an air purification system which utilizes multiple spray nozzles which spray fine mist water droplets that capture airborne particles and a low density water trap to which the water droplets with captured particles adhere for eventual removal.